The orexins (hypocretins) comprise two neuropeptides produced in the hypothalamus: the orexin A (OX-A) (a 33 amino acid peptide) and the orexin B (OX-B) (a 28 amino acid peptide). Two orexin receptors have been studied in mammals, both belonging to the super family of G-protein coupled receptors. The orexin-1 receptor (OX1 or OX1R) is selective for OX-A and the orexin-2 receptor (OX2 or OX2R) is capable to bind OX-A as well as OX-B. The physiological actions in which orexins are presumed to participate are thought to be expressed via one or both of OX1 receptor and OX2 receptor as the two subtypes of orexin receptors.
Orexins also regulate states of sleep and wakefulness, opening potentially novel medicament based therapeutic approaches for narcoleptic or insomniac patients. Orexin receptors are found in the mammalian brain and may have numerous implications in pathologies related to general orexin system dysfunction. Prior art compounds for the treatment of orexin system dysfunction are shown in, for example WO2008008517 and WO2010122151. The structures of the compounds contained in these prior art disclosures are entirely different to the structures disclosed below.
The most advanced orexin receptor antagonists are almorexant, suvorexant (MK-4305) and MK-6096; these compounds are dual orexin receptor antagonists (DORA), displaying similar affinities for both OX1 and OX2 receptor subtypes. These compounds have been shown to be effective in promoting sleep in preclinical species, reducing active wake and increasing both non-REM and REM sleep in rodent and dog studies. In a Phase II clinical trial, almorexant dose-dependently increased sleep efficiency in patients with primary insomnia and tended to improve wake after sleep onset and reduce the latency to persistent sleep. Similarly, suvorexant increased sleep efficiency in a Phase IIb study; significant dose-related effects were also observed for sleep induction and maintenance parameters, and has shown efficacy in several Phase III trials in primary insomnia patients. In both cases, DORAs were well tolerated. MK-6096 shows efficacy in preclinical sleep models and has completed a Phase II clinical trial evaluating efficacy in the treatment of patients with primary insomnia.
Orexins have been found to stimulate food consumption in rats, suggesting a physiological role for these peptides as mediators in the central feedback mechanism that regulates feeding behaviour. Orexin-A is believed to be an endogenous regulator of appetite and an OX1 receptor antagonist reduces food intake in rats. Antagonists of the orexin-A receptor(s) may therefore be useful in the treatment of obesity and diabetes via regulation of appetite. Orexin receptor antagonists may also be useful in the treatment of binge eating disorder and other compulsive disorders including impulse control disorders. The search for new therapies to treat obesity and other eating disorders is an important and increasing challenge, particularly in westernised societies. Current treatments for obesity and diabetes are often either ineffective or show unacceptably high toxicity.
Orexinergic neurons project from the lateral hypothalamus to the ventral tegmental area, amygdala, nucleus accumbens and pre-frontal cortex, indicating a role in reward pathways. Orexin receptor antagonists have been shown to ameliorate sensitisation, withdrawal and self-administration of a number of drugs of abuse in animal models, suggesting that such compounds could have utility in the treatment of dependence and addiction. Many of the regions innervated and modulated by orexin neurons are dopaminergic, suggesting an interplay between these neurotransmitter systems. Antagonists of orexin receptors could modulate central dopaminergic transmission and be effective in the treatment of movement disorders in which dopamine levels are abnormal.
Orexin peptides also play a role in arousal, vigilance and in centrally-controlled autonomic function and have been shown to be involved in the development of a panic-prone state in rodents, an effect that could be reversed by orexin receptor antagonists, suggesting that these would have utility in the treatment of anxiety and panic. Furthermore, levels of beta-amyloid, the pathological hallmark of Alzheimer's disease, were increased by sleep deprivation and orexin peptide infusion in mice. In contrast, levels of beta-amyloid and amyloid placque formation were reduced in amyloid precursor protein transgenic mice upon treatment with an orexin receptor antagonist, suggesting that such compounds may have utility in the treatment of Alzheimer's disease and other dementia-related cognitive dysfunction.
MK-6096 has also completed a Phase II trial evaluating safety and efficacy versus placebo for preventing migraines in participants with episodic migraine. This observation suggests that orexin receptor antagonists may have utility in the treatment or prevention of migraine or other headache disorders, such as tension-type headache, cluster headache, other trigeminal autonomic cephalalgias, other primary headaches such as hemicrania continua or those listed in the International Headache Society 2nd Edition of The International Headache Classification (ICHD-2), secondary headaches such as those listed in ICHD-2, cranial neuralgias such as trigeminal neuralgia or those listed in ICHD-2, or other causes of headache, cranial neuralgia, central or primary facial pain such as those listed in ICHD-2.
A Phase II trial of MK-6096 is currently recruiting to evaluate the safety and efficacy as an adjunctive therapy in participants with major depressive disorder and partial response to antidepressant monotherapy. This observation suggests that orexin receptor antagonists may have utility in the treatment of depression and related disorders.
MK-6096 has also completed a Phase II trial evaluating safety and efficacy in the treatment of painful diabetic neuropathy (PDN) in adults. This observation suggests that orexin receptor antagonists may have utility in the treatment of neuropathic pain or other pain disorders.
Whilst there has been disclosure of related compounds in the prior chemical art, there has been no disclosure of chemically related compounds as antagonists of orexin receptors or for the treatment of orexin system dysfunction.